Let’s get started with the 5th most dangerous PC Virus existed in our Windows operating system.

5. Code Red

The Code Red worm was a computer worm observed on the Internet on July 13, 2001. It attacked computers running Microsoft’s IIS web server. The most in-depth research on the worm was performed by the programmers at eEye Digital Security. They also gave the worm the phrase “Hacked By Chinese!” with which the worm defaced websites. Although the worm had been released on July 13, the largest group of infected computers was seen on July 19, 2001. On this day, the number of infected hosts reached 359,000.

6. CIH

CIH, also known as Chernobyl or Spacefiller, is a computer virus written by Chen Ing Hau of Taiwan. It is considered to be one of the most harmful widely circulated viruses, overwriting critical information on infected system drives, and more importantly, in some cases corrupting the system BIOS.

7. Klez

Klez is a computer worm that propagates via e-mail. It first appeared in the end of 2001. A number of variants of the worm exist. Klez infects Microsoft Windows systems, exploiting vulnerability in Internet Explorer’s Trident layout engine, used by both Microsoft Outlook and Outlook Express to render HTML mail.

8. Melissa

The Melissa worm, also known as “Mailissa”, “Simpsons”, “Kwyjibo”, or “Kwejeebo”, is a mass-mailing macro virus, hence leading some to classify it as a computer worm. First found on March 26, 1999, Melissa shut down Internet mail systems that got clogged with infected e-mails propagating from the worm. Melissa was not originally designed for harm, but it overflowed servers and caused unplanned problems.

9. Sasser

Sasser (sometimes known as the Big One) is a computer worm that affects computers running vulnerable versions of the Microsoft operating systems Windows XP and Windows 2000. Some machines running Windows 98 were infected. Like other worms, Sasser spreads by exploiting the system through a vulnerable network port. Thus it is particularly potent in that it can spread without user intervention, but it is also easily stopped by a properly configured firewall or by downloading system updates from Windows Update. Sasser was first noticed and started spreading on April 30, 2004. This worm was named Sasser because it spreads by exploiting a buffer overflow in the component known as LSASS (Local Security Authority Subsystem Service) on the affected operating systems.

10. Bagle

Bagle (also known as Beagle) is a mass-mailing computer worm written in pure assembly and affecting all versions of Microsoft Windows. The first strain, Bagle.A, did not propagate widely. A second variation, Bagle.B is considerably more virulent. Bagle uses its own SMTP engine to mass-mail itself as an attachment to recipients gathered from the victim computer.

11. Win32/Simile

Win32/Simile (also known as Etap) is a metamorphic computer virus written in assembly language for Microsoft Windows. The virus was released in the most recent version in early March 2002. It was written by the virus writer Mental Driller. Some of his previous viruses, such as Win95/Drill (which used the Tuareg polymorphic engine), have proved very challenging to detect.

12. Nimda

Nimda is a computer worm, isolated in September 2001. It is also a file infector. It quickly spread, eclipsing the economic damage caused by past outbreaks such as Code Red. Multiple propagation vectors allowed Nimda to become the Internet’s most widespread virus/worm within 22 minutes. Due to the release date, some media quickly began speculating a link between the virus and Al Qaeda, though this relationship ended up being untrue. Nimda affected both user workstations (clients) running Windows 95, 98, Me, NT, or 2000 and servers running Windows NT and 2000. The worm’s name spelled backwards is “admin”.

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